A normal ear transmits sounds as shown in FIG. 1 through the outer ear 101 to the tympanic membrane (eardrum) 102, which moves the ossicles of the middle ear 103 (malleus, incus, and stapes) that vibrate the oval window and round window openings of the cochlea 104. The cochlea 104 is a long narrow organ wound spirally about its axis for approximately two and a half turns. It includes an upper channel known as the scala vestibuli and a lower channel known as the scala tympani, which are connected by the cochlear duct. The cochlea 104 forms an upright spiraling cone with a center called the modiolar where the spiral ganglion cells of the acoustic nerve 113 reside. In response to received sounds transmitted by the middle ear 103, the fluid-filled cochlea 104 functions as a transducer to generate electric pulses which are transmitted to the cochlear nerve 113, and ultimately to the brain.
Hearing is impaired when there are problems in the ear's ability to transduce external sounds into meaningful action potentials along the neural substrate of the cochlea 104. To improve impaired hearing, various types of hearing prostheses have been developed. For example, when a hearing impairment is related to the operation of the middle ear 103, a conventional hearing aid or a middle ear implant (MEI) device may be used to provide acoustic-mechanical vibration to the auditory system.
FIG. 1 also shows some components in a typical MEI arrangement where an external audio processor 111 processes ambient sounds to produce an implant communications signal that is transmitted through the skin by an external transmitter 107 to an implanted receiver 108. The receiver 108 includes a receiver coil that transcutaneously receives the implant communications signal which is then demodulated into transducer stimulation signals which are sent by leads 109 through a surgically created channel in the temporal bone to a floating mass transducer (FMT) 110 secured to the incus bone in the middle ear 103. The transducer stimulation signals cause drive coils within the FMT 110 to generate varying magnetic fields which in turn vibrate a magnetic mass suspended within the FMT 110. The vibration of the inertial mass of the magnet within the FMT 110 creates vibration of the housing of the FMT 110 relative to the magnet. This vibration of the FMT 110 is typically coupled to the incus in the middle ear 103 and then to the cochlea 104 and is perceived by the user as sound. See U.S. Pat. No. 6,190,305, which is incorporated herein by reference.
Alternatively, an engagement member of the FMT 110 can be pushed against the round window membrane of the cochlear outer surface as shown in FIG. 2. The FMT 110 has an inner end 203 and an outer end 204 that are connected by a center axis 207. A conical cochlear engagement member 202 is located at the inner end 203 of the FMT 110 with a cochlear engagement surface that couples the mechanical stimulation signal to the round window membrane 201 in the outer cochlear surface. The FMT 110 is pressed against the round window membrane 201 by a fascia piece 205 made of cartilage that is filled into the space between the FMT 110 and the temporal bone 206 of the middle ear which acts as a fixing anatomical structure. The fascia piece 205 is biocompatible and possesses suitable damping properties for stabilizing the FMT 110 in place against the round window membrane 201 and to prevent it from wandering out of place. But this approach depends very much on the exact execution of the filling of the fascia piece 205, which is manually cut to size by the surgeon and yields non-reproducible results. In addition, exerting too much or too little pressure on the round window membrane 201 can yield a distorted sound percept by the patient. Preliminary studies have shown that the preload force on the FMT 110 should lie between 10 and 20 mN to optimally couple the FMT 110 to the round window membrane 201.
U.S. Pat. No. 9,191,760 shows a loading spring 302 in the form of an eccentric spring device, but in that arrangement, the center of gravity 304 of the loading spring 302 does not lie on the center axis 303 of the FMT 110 and the round window engagement member 301. Furthermore, the structural composition of the spring element results in a loading force 305 which is not collinear with the excitation force of the FMT, i.e. the central axis 303 of the FMT 110. Due to these two spring properties, the active FMT 110 will not only move back and forth along its central axis 303, but will also show a rotational component 306 that is offline from the center axis 303. This decreases the vibrational energy introduced into the auditory system by the vibrational oscillations.